A. Muñoz-Escalona

Rheological behaviour of metallocene catalysed high density polytheylene blends

Abstract Dynamic and steady state flow measurements and shrinkage experiments have been carried out to investigate the influence of blend composition on viscous and elastic properties of a new type of polyolefin: high density polyethylenes of high and low molecular weight obtained via metallocene catalysts. The polyethylenes are characterized by narrow molecular weight distribution and absence of branching. A model based on the additivity of molecular weight and free volume has been used to fit the complex and steady state viscosities vs composition data, at constant frequency or shear rate, which show a large positive deviation from linearity. The single correlation between G′ and G″ and the small effect of blend composition on dimensional stability correspond to the behaviour expected for a miscible blend. ‘Sharkskin’ and ‘slip-stick’ effects observed in extrusion process of the high molecular weight sample disappear for blends of 50% and higher content of low molecular weight polyethylene.

The effect of chain architecture on sharkskin of metallocene polyethylenes

Dynamic viscoelastic and extrusion capillary results of metallocene based polyethylenes are analyzed. Three samples show very high viscosities at low frequencies and large relaxation times, which is a symptom of the presence of small amounts of long chain branching (LCB). A linear correlation is found between the sharkskin dynamics (periodicity) and a characteristic entanglement-disentanglement time. It is found that this correlation does not hold for samples suspected of LCB.

Ab initio calculation of ethylene insertion in zirconocene catalyst systems: A comparative study between bridged and unbridged complexes

Abstract The ab initio method at the Hartree-Fock level has been used on two zirconocene systems, the unbridged Cp 2 ZrCH 3 + and the bridged (Si(CH 3 ) 2 )Cp 2 ZrCH 3 + compounds. For each complex the insertion of an ethylene molecule has been followed from reactant to product through the corresponding transition states. The calculated activation energies are of the order of magnitude of those obtained experimentally, explaining the higher activity found for the unbridged catalytic complex compared to the bridged. The geometries are in agreement with other studies and comparable with those derived by crystallographic methods. The α-agostic interaction observed in the reactants as well as in the transition states and resulting products confirms that the Brookhart-Green mechanism is followed for the polymerization reaction.

Effect of a second ethylene molecule on the insertion of ethylene in zirconocene catalyst systems: A QM semiempirical study

The PM3(tm) semiempirical method was used to show the effect of a second ethylene molecule in the backside position on the frontside ethylene insertion in the Cp2ZrnPr+ γ-agostic resting state. The same calculations without a companion second ethylene molecule were performed to compare geometrical parameters, energies, and electrostatic charges. The results obtained show that the geometrical parameters for both cases were identical, but differences in the electrostatic charges were observed. The energy profile presented two barriers, the first corresponding to the alkyl-chain rotation along the ZrCα bond and the second relating to the insertion process itself. The presence of a companion second ethylene molecule affected the energetic profile by lowering the energy barrier of the first stage with respect to the process without the companion second ethylene molecule. These results provide some theoretical support to the well-known trigger effect.

Computational studies of the Brookhart's type catalysts for ethylene polymerization. 1. Effect of the active site conformations on the catalyst activities

Abstract DFT calculations have been carried out on the cationic species for the two different Brookharts catalyst systems: [{ArNCH–HCNAr}NiR″]+ (3a) and [{ArNCMe–MeCNAr}NiR″]+ (3b) (where Ar={2,6-C6H3(Me)2} and R″=Me). These calculations reveal that the conformation of aryl groups attached to nitrogen atoms could provide a suitable explanation for the large experimental differences found in the ethylene polymerization activity.

Temperature and branching dependence of surface extrusion instabilities in metallocene catalysed polyethylene

We have found a branching dependence of critical extrusion temperature in metallocene-catalysed linear and branched polyethylene, below which the commonly found surface distortion instabilities disappear. The elimination of these extrusion instabilities has been observed in a wide range of temperatures up to 23°C above the melting point of each polymer. However, in contrast to previous observations, a window of minimum flow resistance, i.e., extrusion pressure, is not detected. This low temperature effect is discussed in terms of a flow ordered phase at the wall that induces local chain orientation. This so-called mesophase would prevent the mechanism for the formation of distortions to operate below a critical temperature.

Ab initio study of ethylene insertion into M–C bonds of alkylamidinates complexes of group IV ({R′NCRNR′}2MCH3+, M=Zr, Ti, R=H, Ph and R′=H, SiMe3)

Alkylamidinate complexes have been recently reported to be useful catalysts for olefin polymerisation as an alternative to metallocene systems. The present work reports theoretical calculations performed at DFT level for ethylene insertion reactions in zirconium and titanium alkylamidinate compounds with variable structural complexity. The energy barriers obtained for these reactions show that these systems can be considered as efficient olefin polymerisation catalysts, but less active than their metallocene counterparts, in agreement with experimental findings. A comparison between alkylamidinates of Ti and Zr is also provided.